My goal in life has always been to live at least 100 years. There is way too much interesting stuff happening in the world, and I want to take in as much of it as possible before my spark returns to the collective. Lack of sleep can make you look older, feel older, and live a shorter life (“…sleeping less than six hours a night makes you 12% more likely to die prematurely than someone who sleeps up to eight hours…,” more than 9 hours was also associated with shortened lifespan). And so, I sleep. I try to get between 7 and 8 hours per night. Sleep is so important to me that I wrote an extensive article about it two seasons ago. Take a diversion here and read that article defining the many benefits of sufficient sleep to athletes:

“Not only do athletes need sleep to improve on their athletic skills, but the restoration that occurs within muscles during deep sleep is important.” (See article for citation.)

The question is, what happens during sleep that confers all these magical benefits? The answer is, your body and mind appear to go into various reparative states which fix so much which was wrong with you that you didn’t even know! But its not clear that scientists know how it works. Here is a summary about what happens during sleep from IFL Science, with some links from other sources added for good measure:

Stage 1 happens during the first 5-10 minutes of the sleep cycle. This is when it is easiest to be awoken and your muscles begin to relax. This is also when you are most likely to experience a hypnagogic jerk, when muscles suddenly twitch, or you may feel like you are falling.

Stage 2 takes up half of the sleep cycle, lasting around 45-50 minutes. If you are woken up at this time, you will likely feel very disoriented. Breathing becomes relaxed and slowed as the body temperature drops, giving your heart a bit of a break, Schocker explains. Neural activity also decreases during this time, as the body prepares for the next phase of the cycle.

Stage 3 is when deep sleep occurs for about 20 minutes and the body is given the chance to restore itself. Brain waves become slow and steady, muscle and tissues are repaired (which is why proper sleep is a crucial factor when building muscle), and various hormones are secreted around the body. Waste management systems in the brain become very active, flushing out that which impairs cognitive function. Despite being the time when most rejuvenation happens, this is also the phase in which someone is most likely to walk or talk in their sleep. It is incredibly hard to wake someone up during this phase, as anyone who has dealt with a sleepwalker can attest to.

Rapid Eye Movement (REM) is the final stage in the cycle, lasting about 20 minutes. Though the eyes are closed, they move very quickly underneath the eyelid. Brain activity is high and the bulk of dreaming takes place during this time. However, muscles are paralyzed, preventing voluntary muscle movement.

Memory consolidation and growth hormone release happen in earnest during NREM. … During sleep, the body has a chance to replace chemicals and repair muscles, other tissues and aging or dead cells. Growth hormones are released during deep sleep. —Sleepdex, Why we Sleep.

So, while we may not know exactly how the effects of sleep confer all the benefits listed, we know that a person sleeping between 6 and 8 hours per night gets the most of those benefits. (To reiterate, those benefits include: improved cognition, improved health, increased life expectancy, improved athletic performance, and increased sex drive.) In case you’re wondering, you can’t really “catch up” on sleep, either; “Sleep is not a bank.”